Water cooled internal combustion engines are in widespread use in motor vehicles such as automobiles and trucks. Such vehicles have a radiator which acts as a reservoir for the cooling fluid or coolant, which generally is a mixture of water and anti-freeze. A cooling fan is used to move air through the radiator for the purpose of cooling the fluid which, during operation of the vehicle, continuously circulates through the radiator. Although the operation of the cooling fan is necessary when the engine is hot, such as occurs when the vehicle is stationary or is moving at low speeds in relatively high ambient temperatures, much of the time operation of the fan is not required.
When the vehicle engine initially is started, the engine is cold and operation of the cooling fan prior to the time the engine warms up to its operating temperature clearly is not necessary. In addition, when a vehicle is operating at higher speeds, such as ordinarily are encountered in highway driving and even in most aspects of city driving, sufficient air passes through the radiator to cool it without the fan operation. This is known as ram air cooling.
With increasing concern over the efficiency of motor vehicle operation and, in addition, concern over the high cost of fuel for operating such vehicles, substantial effort is being devoted to improving the number of miles travelled for each gallon of fuel consumed. It is known that radiator cooling fans require a substantial amount of horsepower to rotate them. This is particularly true of large engine motor vehicles such as trucks which require large radiators and correspondingly large cooling fans. Several horsepower of energy are consumed in the rotation of the fans for such vehicles. Thus, if the fan is operating or being rotated by the engine when it is not necessary to effect cooling of the engine, a clear waste of engine horsepower results. In addition, when an engine is cold, it is not desirable to increase the flow of air through the radiator since this tends to lengthen the time required to heat the cooling fluid up to the desired operating temperature of the engine. Thus, it has been recognized that it is highly desirable to rotate the radiator cooling fan only when the temperature of the engine coolant is hot enough to require the operation of the cooling fan to draw air through the radiator.
In the past, some motor vehicle cooling fans have been mounted on a shaft which is an extension of the water pump shaft through a type of clutch formed by material which permits slippage between the shaft and the fan hub when the engine is cold and which provides a relatively firm engagement between the shaft and the fan hub when the engine is hot. This arrangement, however, is not particularly efficient; and even when the engine is cold, there is a considerable amount of friction between the hub and the shaft, therefore resulting in the unnecessary consumption of energy even under cold operating conditions.
Other arrangements have been made for engaging and disengaging a freely rotatable fan at different engine speeds. These arrangements, however, since they do not sense the primary condition, that is temperature of the engine coolant, upon which fan operation should be based, are unsatisfactory.
Electromagnetically operated clutches for engaging and disengaging the fan from the engine rotation have been employed in the past. Because of the location of the cooling fan; and, in addition, because of the belts and pulleys located in the forward area of the engine, the implementation of electromagnetic clutch assemblies for use with the radiator cooling fan has been difficult. Systems have been used which modify the conventional water pump cooling shaft to an extended configuration to permit mounting of an electromagnet field coil and clutch assembly on the shaft. In some cases, the field coil is mounted for rotation with the shaft which requires the use of slip rings for supplying electrical energy to the coil. Other structures employ additional bearings, binding rings, seals and the like for mounting the field coil on the shaft and then include additional apparatus to attach the field coil to a bracket which in turn is attached to the engine. The weight of the coil, however, is supported on the water pump shaft as is the weight of the fan and its hub and bearing assembly. All this additional weight creates a strain on the water pump shaft and, in the event of a water pump failure, which is relatively common in motor vehicles, disassembly of a large number of parts is required. As a consequence, the repair or replacement of the water pump, in the event of failure, is increased in cost because of the disassembly and reassembly required for the additional components used to electromagnetically operate the fan engagement and disengagement.
It is desirable to provide an electromagnetically operated clutch for engaging and disengaging the cooling fan in response to coolant temperature variations in a simple and effective manner. Ideally, this should be done with minimum modification to existing water pump/cooling fan structures in an inexpensive manner which also does not impair or complicate the removal and replacement of water pump parts in the event repair of such parts becomes necessary.